1. Field of the Invention
This invention relates to forming passivated surfaces on solid materials and more particularly to formation of passivated surfaces on semiconductor materials by laser irradiation.
2. Brief Description of the Prior Art
The design and manufacture of semiconductor devices is based on the electronic properties of the bulk semiconductor material of which they are made. The bulk material is ordinarily a single crystal having a uniform lattice structure and containing various dopants, which introduce the impurity levels required to provide the desired electrical conduction properties. Within the bulk crystal each atom is bound to its neighbors in the lattice. However, at the surface of the crystal the regular lattice is interrupted. Accordingly the atoms at the surface of the crystal exhibit dangling bonds, not used to join neighboring atoms, but available to react with other materials in the environment, e.g., oxygen, to introduce surface energy levels or states that may change or interfere with the electrical properties of the device. In order to prevent such degradation of the properties of the semiconductor device, a passivating layer is customarily applied to the surface of the crystal in order to tie up any dangling bonds and thereby stabilize the surface against any further degradation.
In the case of silicon, which has been the material of choice for a wide variety of electronic applications for almost four decades, the surface is easily passivated by formation thereon of a stable adherent layer of silicon dioxide. The passivating oxide layer can be formed by conventional techniques such as thermal oxidation of the surface or chemical vapor deposition (CVD). This stable surface chemistry of silicon, as well its low cost, easy availability, and mature processing technology, has favored the wide use of silicon as the substrate material in the manufacture of semiconductor devices. However, silicon also has certain limitations, particularly in the field of wireless communications, especially as higher frequencies have come into use. Because silicon has an indirect band gap structure, it is less efficient for applications involving radio-frequency (RF) and wireless communications. Consequently, other semiconductor materials, especially those possessing a direct band gap (i.e., those materials wherein the valence band energy maxima and conduction band energy minima occur at the same k value in the E-k space) have come to be used for such applications.
Among the direct-band gap semiconductors, gallium arsenide (GaAs), a III–V compound semiconductor, has come to be widely used. As a direct consequence of the direct band gap, together with a higher band gap energy (1.42 eV for GaAs, versus 1.1 eV for Si) GaAs supports high frequency device applications, such as cellular (e.g., cellular telephones (cell phones)) and other wireless communication equipment, whereas silicon cannot. Consequently, GaAs semiconductor devices, although generally more expensive, have found a commercially valuable niche where silicon cannot effectively compete.
However, manufacture of semiconductor devices from GaAs also faces some challenges. Most importantly, unlike Si, GaAs does not form a natural and stable protective passivating layer. Consequently, it has been difficult to design and manufacture metal-oxide-semiconductor (MOS) devices using GaAs. Furthermore, the oxides of gallium and arsenic are somewhat volatile, with the result that they may escape from the surface after they are formed, causing further depletion of their respective atoms with time. Since the electronic properties of GaAs depend on the stoichiometric ratio of Ga to As atoms, such an uncontrolled oxidation degrades the electronic performance of devices fabricated on GaAs wafers. To overcome this problem, passivation of the surface is conventionally achieved by treating the surface with a protective overlayer, generally an organic polymer. Although the passivating overlayer protects the surface, and hence stabilizes the electronic properties of the GaAs, it also introduces an undesirable feature because it entails a higher cost for controlling the electronic properties of the device.
Accordingly, a need has continued to exist for a method of preparing a stable passive surface on gallium arsenide wafers and the like used in manufacturing semiconductor devices.